Atomic absorption spectrometer

ABSTRACT

An atomic absorption spectrometer is disclosed which includes a monochromater ( 30 ) and an optical path defined by a toric mirror ( 14 ), a flat mirror ( 16 ), a flat mirror ( 20 ), a flat mirror ( 24 ), a toric mirror ( 22 ) and a further toric mirror ( 26 ). The toric mirror ( 26 ) directs light through entrance slit ( 32 ) of the monochromater ( 30 ) so that radiation is reflected from diffraction grating ( 38 ) and out exit slit ( 40 ) to a detector ( 42 ). A sample stage ( 18 ) in the form of a furnace is located between the mirrors ( 16 ) and ( 20 ). The monochromater is oriented so that the entrance slit is arranged transverse to the vertical. In one arrangement, a magnifying means is also provided which magnifies an image of the aperture ( 32 ) at the sample station ( 18 ) to increase the amount of radiation which is focused at the sample stage, and which is passed through the aperture to the detector or a focusing element provided by one of the mirrors may be provided for focusing radiation at a location other than sample station ( 18 ) so an enlarged out of focus image of the entrance slit ( 32 ) is produced at the sample station ( 18 ) to increase the amount of radiation which passes through the slit and is received by the detector ( 42 ).

FIELD OF THE INVENTION

This invention relates to an atomic absorption spectrometer.

BACKGROUND ART

Atomic absorption spectrometers are well known and analyse a samplematerial by directing a beam of electromagnetic radiation through asample and then detecting absorption of the beam by the sample andtherefore the concentration of the sample.

Atomic absorption spectrometers usually carry a carousel of hollowcathode lamps which are selectively placed into alignment with theoptical axis of the instrument to enable electromagnetic radiation of aparticularly wavelength to be directed to a sample. The optical path ofthe instrument generally comprises an array of lenses or mirrors and asample stage interposed in the optical path through which theelectromagnetic radiation passes. Electromagnetic radiation is directedto a monochromater which includes a monochromater mirror for reflectingthe radiation to a diffraction grating which reflects the radiation backto the mirror. The mirror reflects the radiation to a detector foranalysis. The diffraction grating can be moved under computer control totune the instrument to the particular wavelength which is emitted by thecathode lamp. In general, the instrument will step through a number ofanalysis steps, each using a different cathode lamp to provide radiationof different wavelength which passes through the sample. The diffractiongrating is moved under computer control so as to reflect that wavelengthback to the monochromater mirror for reflection to the detector. Theabsorption of that wavelength by the sample and therefore the lack ofdetection of that particular wavelength by the detector indicates thatthe sample does include atoms of a particular type which absorb thatwavelength and therefore constituents of the sample can be identified.

The light which enters the monochromater passes through a vertical slitfor receipt by the monochromater mirror and after reflection by thediffraction grating, and the monochromater mirror towards the detector,passes through another vertical slit to be received by the detector.

The cathode lamps which produce the radiation generally comprise a 3 mmdiameter source which is directed by the optics of the instrument tofocus at the sample to provide a 3 mm diameter source image at thesample. The radiation then passes to the slit in the monochromater andpasses through the slit into the monochromater. Typically the slit isabout 0.25 mm wide. An image of the slit or, in other words, the imagewhich is received by the detector when focused at the source istherefore a vertical slit of about 0.25 mm width. Radiation produced bythe cathode tube and which falls outside the perimeter of the slit istherefore not received by the photodetector of the spectrometer, andtherefore plays no function in analysis of the sample material. Thus, asignificant amount of 3 mm diameter spot of light at the sample stage islost.

In order to provide sample material for analysis, the spectrometerincludes a burner which produces a flame to ionize sample material whichis introduced into the flame. The ionized sample material in general iscarried up with the flame in the burner and the radiation from thecathode tube is focused in the flame at the sample position so that theradiation, more likely than not, will pass through atoms of the samplematerial and be absorbed. By detecting absorption of the radiation, theconstituents of the sample material can be measured as is describedabove.

In general, because the sample is ionized in a flame, the atoms of thesample will move upwardly with the flame and will pass through the imageof the slit of the monochromater at the sample stage therefore fallingwithin the path of the radiation which passes through the source imageand therefore through the slit in the monochromater for detection.

However, if other methods of producing sample material are utilized, thelikelihood of the radiation from the source passing through sample atomscan be much less. For example, if sample is produced in a graphitefurnace rather than a flame, there is a significant possibility thatsample atoms will not locate in the source image of the slit at thesample location and therefore will not fall within the path of theradiation which is actually detected by the detector. Thus, there is apossibility that sample atoms and therefore the true constituent natureof the sample material will not be determined.

Graphite furnaces generally comprise a graphite tube of circularcross-section which is located at the sample stage. The graphite tube isopen at both ends and the radiation passes through the tube. Highelectric current is supplied across the graphite tube to heat thegraphite tube and therefore atoms of sample material which is depositedin the tube. In general, the sample material is deposited in the tube bya very thin needle which passes through an aperture or bore in theinstrument and through an aperture or bore in the graphite tube. Withconventional instruments, considerable skill is required in order todeposit the sample material at a correct location so that when thegraphite furnace is energized, sample atoms will rise in the graphitefurnace through the source image of the slit and therefore in the pathof radiation which is actually detected by the detector.

If the sample material is not deposited centrally in the graphitefurnace, but slightly to one side, the possibility exists that when thegraphite furnace is heated, the sample atoms will travel verticallyupwards and miss the source image of the slit and therefore not fallwithin the path of radiation which is actually detected by the detector.Thus, those sample atoms will not be detected, thereby resulting inimproper or, in fact, no analysis result of the sample material.

The slit in the monochromater which allows the radiation to pass intothe monochromater is required in order to block out unwanted wavelengthsand also to prevent the detector from detecting extraneous radiationwhich may completely smother wavelengths which the detector desires todetect. In particular, with a graphite furnace, since the graphitefurnace is heated to high temperature and glows white hot, the slit isrequired to be positioned so that it does not allow imaging of radiationproduced from the graphite furnace itself onto the detector, which wouldotherwise saturate the detector and prevent proper analysis of radiationwhich passes through the sample and which is produced by the cathodetube. To prevent extraneous radiation from being detected by thedetector, the size of the slit is changed depending on the wavelengthbeing detected and also, in some instances, the slit is masked to reducethe length of the slit to ensure that radiation which is produced by thegraphite furnace itself is not received by the detector.

Thus, the fact that the masked slit only allows part of the radiationwhich passes through the sample to enter the monochromater and theorientation of the slit can therefore greatly reduce the sensitivity ofthe spectrometer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an atomic absorptionspectrometer which overcomes the above problem.

The invention may be said to reside in an atomic absorption spectrometerincluding:

-   -   a monochromater;    -   an optical path for receiving radiation from a source and for        directing the radiation to the monochromater;    -   a sample station in the optical path;    -   the monochromater having an entrance slit and a diffraction        grating;    -   a detector for receiving radiation from the monochromater; and    -   the monochromater being oriented such that the entrance slit is        arranged transverse to the vertical so that an image of the slit        at the sample station, if radiation passes from the detector        along the optical path, is transverse to the vertical so that        sample materials produced in a graphite furnace will move        upwardly from a lower portion of the graphite furnace and across        the transversely arranged image of the slit so that radiation        emanating from the source and travelling along the optical path        will pass through the sample material and be detected by the        detector.

Since the monochromater is oriented such that the entrance slit isarranged transverse to the vertical, sample material which is loadedinto the graphite furnace and which merely sits on the lower internalsurface of the furnace, will produce sample atoms when the furnace isenergized which will pass upwardly, and must cross over the image of theslit and therefore will be located in the path of the radiation which isactually detected by the detector. Thus, the sample material will bedetected rather than missing the radiation which is detected by thedetector thereby ensuring more complete analysis of sample material andgreater sensitivity of the instrument.

Preferably the entrance slit is arranged horizontal or perpendicular tothe vertical.

In one embodiment of the invention the optical path includes magnifyingmeans for magnifying the beam of radiation so that the image of the slitat the sample stage has an increased dimension in the directionperpendicular to the horizontal, or in other words, in the verticaldirection, thereby increasing the amount of time that sample atoms passthrough the radiation which is detected by the detector and furtherincreasing the sensitivity of the spectrometer.

In the preferred embodiment of the instrument the optical path isdefined by a plurality of mirrors, some of which are toric mirrors fordirecting the radiation from the source, to the sample stage and then tothe entrance slit of the monochromater.

Preferably the magnifying means is comprised of a first portion of theoptical path in which the radiation travels from the sample stage to abeam focusing element, being greater than the length of a second portionof the optical path from the focusing element to the slit.

Preferably the radiation emanating from the source is focused at thesample stage by a primary focusing element.

Preferably the primary focusing element includes at least one toricmirror.

Preferably the beam focusing element comprises a further toric mirror.

Preferably the monochromater also includes a monochromater mirror forreflecting radiation received through the slit to the diffractiongrating and for receiving radiation reflected from the diffractiongrating, an exit slit in registry with the detector so that theradiation reflected from the monochromater mirror after reflection fromthe diffraction grating passes through the exit slit to the detector.

The invention may also be said to reside in an atomic absorptionspectrometer including:

-   -   a monochromater;    -   an optical path for receiving radiation from a source and for        directing the radiation to the monochromater;    -   a sample station in the optical path;    -   the monochromater having an entrance aperture and a diffraction        grating;    -   a detector for receiving radiation from the monochromater; and    -   magnifying means in the optical path for magnifying an image of        the aperture at the sample station if radiation passes from the        detector along the optical path, to thereby increase the amount        of radiation which is focused at the sample stage and which is        then passed through the aperture and received by the detector.

According to this aspect of the invention, since the magnifying meanseffectively magnifies the image of the aperture, more of the radiationwhich is actually focused at the sample stage is eventually received bythe detector to thereby increase sensitivity and reduce the likelihoodthat some of the sample material produced in a graphite furnace will nottravel into the beam of radiation travelling along the optical path fromthe source. Preferably the magnifying means is comprised of a firstportion of the optical path in which the radiation travels from thesample stage to a beam focusing element, being greater than the lengthof a second portion of the optical path from the focusing element to theslit.

Preferably the radiation emanating from the source is focused at thesample stage by a primary focusing element.

Preferably the primary focusing element includes at least one toricmirror.

Preferably the beam focusing element comprises a further toric mirror.

The invention may also be said to reside in an atomic absorptionspectrometer including:

-   -   a monochromater;    -   an optical path for receiving radiation from a source and for        directing the radiation to the monochromater;    -   a sample station in the optical path;    -   the monochromater having an entrance aperture and a diffraction        grating;    -   a detector for receiving radiation from the monochromater; and    -   focusing means in the optical path for producing a focal point        if radiation passes from the detector along the optical path at        a location other than at the sample station and the source so an        enlarged out of focus image of the entrance slit is produced at        the sample station.

According to this aspect of the invention, since the focusing meansfocuses the radiation between the sample station and the source, anenlarged out of focus image of the entrance slit will occur at thesample stage, thereby increasing the amount of radiation which isactually detected by the detector. Although the image of the entranceslit would be out of focus at the sample station, this has no bearing onthe atomic absorption characteristics of the radiation and sample andtherefore, no bearing on the detection of the radiation by the detectorand the analysis by the atomic absorption spectrometer. However, sincethe entrance slit is effectively magnified at the sample station, againthe invention reduces the likelihood that the sample material producedin a graphite furnace at the sample station will not travel into thebeam of radiation detected by the detector.

The location of the focus can be between the sample station and thesource or between the sample station and the monochromater. In thepreferred embodiment of the invention, the focal point is at a locationbetween the sample station and the source.

Preferably the optical path includes at least one toric mirror and thetoric mirror has a curvature such as to create a focus for radiationpassing back from the detector to the source along the optical path, atthe location between the sample station and the source.

This aspect of the invention may be used instead of or in combinationwith the previously described aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of an atomic absorption spectrometerembodying the invention;

FIG. 1A is a cross-sectional view along the line 1A—1A of FIG. 1;

FIG. 2 is a schematic view of an end view of a graphite furnace showingthe image of an entrance slit of a monochromater illustrating thepreferred embodiment of the invention;

FIG. 3 is a view similar to FIG. 2 but of a conventional arrangement;

FIG. 4 is a side view of a graphite furnace which can be used in thepreferred embodiment of the invention;

FIG. 5 and FIG. 6 are views similar to FIGS. 1A and 2 showing a furtherembodiment; and

FIG. 7 is a view of a further embodiment of the invention.

With reference to FIG. 1, the relevant components of the atomicabsorption spectrometers of the present invention are shown in theirpreferred form. The spectrometer includes a hollow cathode lamp 10 forproducing electromagnetic radiation 12 at a predetermined wavelength. Ingeneral, a plurality of lamps 10 will be arranged in a carousel (notshown) and sequentially moved into the position shown in FIG. 1 so thateach can be energized to produce a beam of light 12 of a particularwavelength which will pass through a sample material and then bedetected to enable the sample material to be analysed.

A toric mirror 14 receives the radiation 12 from the lamp 10 andreflects the radiation to a flat mirror 16. The flat mirror 16 reflectsthe radiation to a sample stage 18 and focuses the radiation 12 at thesample stage 18 so the radiation will pass through sample atoms producedat the sample stage 18.

The cathode tubes 10 generally include a 3 mm diameter source andtherefore the radiation focused at the sample stage 18 is generally a 3mm diameter spot at the sample stage 18.

Radiation which passes through the sample stage 18 is reflected by aflat mirror 20 to a flat mirror 24 which then reflects the radiation toa toric mirror 22. The toric mirror 22 reflects the radiation to afurther toric mirror 26 which reflects the radiation to a monochromater30. The monochromater 30 includes an entrance aperture in the form of aslit 32 (best shown in FIG. 1A) which is provided in an end plate 34 ofthe monochromater 30. The monochromater has a monochromater mirror 36which reflects the radiation to a diffraction grating 38 which in turnreflects the radiation back to the monochromater mirror 36 forreflection through an exit slit 40 to detector 42.

The toric mirror 14 focuses the beam of radiation 12 at the sample stage18 and the beam of radiation which passes from the sample stage 18 andreceived by the toric mirror 22 leaves the toric mirror 22 as a parallelbeam of radiation 12′. The beam 12′ is reflected by the toric mirror 26and focused at the entrance slit 32. The beam path from the sample stage18 to the toric mirror 22 is greater than the beam path from the toricmirror 26 to the slit 32, thereby amplifying or magnifying theradiation. Thus, the different length of the beam paths between thefocus at the sample stage 18 and the toric mirror 22 and from the toricmirror 26 to the focus at the slit 32 acts as an amplifier or magnifierwhen viewed back from the detector 42, the purpose of which will bedescribed in more detail hereinafter.

The sample stage 18 may include an apparatus for producing sample atomsof any particular type. However, the present invention has particularapplication to spectrometers which include a graphite furnace 37 for theproduction of sample atoms. The graphite furnace is shown in FIG. 4 inside view and end views of the furnace are shown in FIGS. 2 and 3.

With reference to FIGS. 2 and 4, the furnace generally comprises a tube39 of graphite which has a circular cross section as shown in FIG. 2.The ends 41 and 43 of the tube 39 are open and the radiation 12 can passthrough the tube 39 and be focused at point 18 shown in FIG. 4 whichdefines the sample stage referred to in FIG. 1 and then travels to themirror 20.

The graphite furnace 37 can include dam walls 45 at each end which actto retain sample material deposited in the furnace from flowing out theopen ends 41 and 43.

When the graphite furnace is heated, sample atoms are generated whichpass vertically upward from the sample deposited in the furnace asrepresented by reference S′ in FIGS. 2 and 3 towards the upper portionof the furnace.

As shown with reference to FIG. 1A, the entrance slit 32 is oriented sothat it is substantially horizontal as best shown in FIG. 1A. Thus, theimage of the slit 32 at the sample stage 18 is a horizontal slit 32′ asshown in FIG. 2. Thus, when the sample material S′ passes upwardly inthe furnace 37, the sample material will cross over the slit 32′ therebyensuring that radiation which passes through the sample stage 18 and iseventually received by the detector 42 will in fact pass through thesample material S′ generated by the furnace 37.

When considering an image of the slit 32 at the sample stage 18, it isconventional to think of the image in terms of directing radiation fromthe detector 42 back through the monochromater 30 and along the opticalpath of the beam 12 to the sample stage 18. If radiation or light isshone in that direction from the detector 42, then an image of the slit32 will appear at the sample stage 18 and will effectively define thearea of the sample stage 18 through which radiation actually passes fromthe tube 10 and which is actually detected by the detector 42.

FIG. 3 shows the orientation of the slit 32 of a conventional atomicabsorption spectrometer as imaged at the sample stage 18. It can be seenthat the slit 32′ in

FIG. 3 is vertically oriented and therefore sample material S″,incorrectly deposited in the furnace 37, will generally move verticallyupward as shown by lines S′″ and can well miss the radiation 12 passingthrough the furnace 37 and which will actually be detected by thedetector 42. However, as illustrated with reference to FIG. 2, the factthat the slit 32 in the preferred embodiment is oriented such that theimage is horizontal rather than vertical means that sample material,regardless of where it is deposited in the furnace 37 will rise upwardlyand cross over the slit 32 thereby ensuring that radiation which passesthrough the sample stage 18 and which is actually detected by detector42, will pass through the sample material S′ generated by the furnace37.

In order to orient the slit 32 in the horizontal orientation rather thanthe vertical orientation, the entire monochromater 30 of a conventionalatomic or absorption spectrometer can be thought of as being rotated 90°from its normal position. Thus, not only is the slit 32 arrangedhorizontally, the diffraction grating 32 is also rotated so as to be inalignment with the radiation passing through the slit 32 so that thediffraction grating can separate the wavelength passing through the slit32 and then direct those wavelengths to the mirror 36 for reflectionthrough the horizontal exit slit 40 to the detector 42.

The amplification of the beam 12 produced by the difference in pathlength from the sample stage 18 to the mirror 22, compared to the lengthof the path length from the mirror 26 to the path 32, produces awidening of the image of the slit 32 at the sample stage 18 as shown bydouble-headed arrow W in FIG. 2 so as to produce a much wider image ofthe slit as shown by reference 32″ in FIG. 2. The result of this is thatsample material passing upwardly in the furnace 37 will remain longer inthe image of the slit 32 at the sample stage thereby increasing thelikelihood of impingement with radiation from the tube 10 and thereforeincrease of the sensitivity of the spectrometer.

The increase in width W of the image of the slit 32 means that theactual area through which radiation passes is much closer to the actual3 mm diameter spot of radiation which is focused at the sample stage 18and therefore considerably less radiation is lost than is the case whencompared to conventional arrangements.

Because of the magnification of the image of the slit produced by thedifference in path lengths from the sample stage 18 to the mirror 22,compared to the length of the path from the mirror 26 to the slit 32,the mirror 36 is slightly larger than a conventional monochromatermirror to ensure that it does receive all the radiation passing throughthe slit 32 and therefore reflects all the radiation to the diffractiongrating 38 and then reflects all the radiation from the diffractiongrating 38 to the exit slit 40 for detection by the detector 42.

The preferred embodiment of the invention therefore provides an atomicabsorption spectrometer which is particular useful with a graphitefurnace and which can not only increase sensitivity of the instrumentbut also ensure that radiation which is received by the detector 42 willactually pass through the sample material generated by the furnace whenthe furnace is heated. Thus, more reliable analysis results can beobtained with greater sensitivity of the instrument.

FIGS. 5 and 6 show a second embodiment in which the entrance apertureand exit aperture are in the form of a square 32. The aperture 32results in a magnified image of the square aperture at the sample stage,as is shown in FIG. 6. The configuration of the aperture could be othershapes and the fact that the image of the aperture will be magnified atthe sample stage 18 means more radiation will be received by thedetector 42. The configuration of the aperture 32 will change (as willthe configuration of the slit aperture in FIGS. 1 to 4) under control ofthe spectrometer to filter out unwanted wavelengths from entering themonochromater 30. However, regardless of the shape or size of theaperture, the image of the aperture at the sample stage will be enlargedso more radiation passing through the sample is collected by thedetector 42. The maximum size of the aperture image is preferably withinthe confines of the furnace 37, as shown in FIGS. 2 to 5, so radiationproduced by the heated furnace 37 is not received by the detector 42.

In the embodiment shown with reference to the drawings the mirrors 14and 16 and the mirrors 22 and 24 are arranged such that the reflectionof the beam 12 is generally at right angles. However, in otherembodiments, the mirrors could be arranged such that the reflection isnot at right angles and the radiation is reflected from the mirrors 14and 24 at an obtuse angle with respect to the incoming beam 12 to themirrors 16 and 24. Thus, rather than the beam 12 having a right angledconfiguration at the mirrors 14, 16 and 22, 24, the beam has a zigzaggedshape.

Furthermore, in other embodiments, rather than the toric mirror 14 beingprovided before the flat mirror 16, the order of these mirrors may bereversed as may the order of the mirrors 22 and 24.

FIG. 7 shows a further embodiment of the invention in which likereference numerals indicate like parts to those previously described.

In the embodiment of FIG. 7, the toric mirror 22 is changed so that iflight was to emanate back from the detector 42 along the optical path,the light would be focused at point F, which is not at the samplestation 18, as in the embodiment of FIG. 1 but, rather, at a positionbetween the sample station 18 and the light source 10. Most preferably,the distance between the focal point F and the station 18 would be inthe order of 15 to 25 mm. The focusing of the light at the point Fresults in an enlarged image of the slit 32 at the sample station 8albeit an out of focus image of the slit. However, because the image ofthe slit is enlarged at the station 18, the detector will detect moreradiation passing through the graphite furnace at the station 18 asshown by the image 32″ in FIG. 7. The fact that the image of the slit 32is out of focus at the sample station 18 has no bearing on the atomicabsorption characteristics of the radiation or on the analysis performedby the detected radiation. However, as is apparent from a considerationof FIG. 7, the image of the slit 32 is much larger at the sample stationS than the conventional technique, thereby ensuring that a larger regionof radiation is available in which sample material can move andintercept the light beam from the source 10 travelling to the detector42. Thus, the characteristics of the spectrometer are improved in thesame manner as described with reference to FIGS. 1 to 6.

The embodiment of FIG. 7 can be used instead of or in combination withthe arrangements described with reference to FIGS. 1 to 6.

Since modifications within the spirit and scope of the invention mayreadily be effected by persons skilled within the art, it is to beunderstood that this invention is not limited to the particularembodiment described by way of example hereinabove.

1. An atomic absorption spectrometer including: a monochromater; anoptical path for receiving radiation from a source and for directing theradiation to the monochromater; a sample station in the optical path;the monochromater having an entrance slit and a diffraction grating; adetector for receiving radiation from the monochromater; and themonochromater being oriented such that the entrance slit is arrangedtransverse to the vertical so that an image of the slit at the samplestation, if radiation passes from the detector along the optical path,is transverse to the vertical so that sample materials produced in agraphite furnace will move upwardly from a lower portion of the graphitefurnace and across the transversely arranged image of the slit so thatradiation emanating from the source and travelling along the opticalpath will pass through the sample material and be detected by thedetector.
 2. The spectrometer of claim 1 wherein the entrance slit isarranged horizontal or perpendicular to the vertical.
 3. Thespectrometer of claim 1 wherein the optical path includes magnifyingmeans for magnifying the beam of radiation so that the image of the slitat the sample stage has an increased dimension in the directionperpendicular to the horizontal, or in other words, in the verticaldirection, thereby increasing the amount of time that sample atoms passthrough the radiation which is detected by the detector and furtherincreasing the sensitivity of the spectrometer.
 4. The spectrometer ofclaim 1 wherein the optical path is defined by a plurality of mirrors,some of which are toric mirrors for directing the radiation from thesource, to the sample stage and then to the entrance slit of themonochromater.
 5. The spectrometer of claim 3 wherein the magnifyingmeans is comprised of a first portion of the optical path in which theradiation travels from the sample stage to a beam focusing element,being greater than the length of a second portion of the optical pathfrom the focusing element to the slit.
 6. The spectrometer of claim 1wherein the radiation emanating from the source is focused at the samplestage by a primary focusing element.
 7. The spectrometer of claim 6wherein the primary focusing element includes at least one toric mirror.8. The spectrometer of claim 7 wherein the beam focusing elementcomprises a further toric mirror.
 9. The spectrometer of claim 1 whereinthe monochromater also includes a monochromater mirror for reflectingradiation received through the slit to the diffraction grating and forreceiving radiation reflected from the diffraction grating, an exit slitin registry with the detector so that the radiation reflected from themonochromater mirror after reflection from the diffraction gratingpasses through the exit slit to the detector.